Best practice: How to expose a read-only ICollection

Question

I have an ICollection called foos in my class which I want to expose as read-only (see this question). I see that the interface defines a

Answer 1

I seem to have settled on returning IEnumerable with the objects cloned.

public IEnumerable GetFooseList() {
   foreach(var foos in Collection) {
     yield return foos.Clone();
   }
}

• requires a Clone method on Foos.

This allows no changes in the collection. Remember that ReadonlyCollection is "leaky" since the objects inside it can be changed as mentioned in a link in another post.